In a letter to DNC Chairwoman Debbie Wasserman Schultz on Monday, Sanders' campaign attorney Brad Deutsch argued that a committee set up to fundraise for Clinton's campaign, the DNC and state parties skirted legal fundraising limits in an effort to drive small donations and by paying Clinton's campaign staff with the committee's funds.

The committee, known as the Hillary Victory Fund, is a joint fundraising committee allowed under Federal Election Commission (FEC) rules in which two or more political committees can fundraise together.

"The Hillary Victory Fund has reported receiving several individual contributions in amounts as high as $354,400 or more, which is over 130 times the $2,700 limit that applies for contributions to Secretary Clinton's campaign," Deutsch wrote in the letter. "Bernie 2016 is particularly concerned that these extremely large-dollar individual contributions have been used by the Hillary Victory Fund to pay for more than $7.8 million in direct mail efforts and over $8.6 million in online advertising, both of which appear to benefit only HFA by generating low-dollar contributions that flow only to HFA, rather than to the DNC or any of the participating state party committees."

Under FEC rules, participants in the committee have to divide money raised from each big donor based on a formula they agree upon. For the Hillary Victory Fund, the first $2,700 from each donor goes to Clinton, the next $33,400 goes to the DNC and up to $10,000 is distributed to state parties. Thirty-two state parties signed onto this joint fundraising agreement when it was created last year.

The Sanders campaign said that a large chunk of the money fundraised through the fund has gone directly to Clinton's campaign. According to the Center for Responsive Politics and Politifact, the fund has given $4.4 million to Clinton's campaign, $2.3 million to the DNC and $2.2 million to state political parties.

"While the use of joint fundraising agreements has existed for some time -- it is unprecedented for the DNC to allow a joint committee to be exploited to the benefit of one candidate in the midst of a contested nominating contest," Sanders' campaign manager Jeff Weaver said.

Robby Mook, Clinton's campaign manager, said in a statement Monday that Sanders' campaign's "false attacks have gotten out of hand." On a call with reporters, Mook defended the joint fundraising committee's activities and how the funds are used.

"The FEC has clear rules," he said. "We follow the law and we follow the rules with our joint fundraising committee...The Sanders campaign is trying to, trying to insinuate that something is either not according to the rules or unfair and neither is true."

The letter to the DNC noted that The Washington Post published a story in February about Clinton's fund that quoted a Sanders campaign adviser saying what Weaver said Monday, though the campaign did not take action on the issue at the time. Deutsch and the campaign failed to note Monday that Sanders agreed to set up his own joint fundraising committee with the DNC late last year, but it hasn't been active.

The Sanders campaign is also directing its complaints to the DNC instead of the FEC, which reviews issues like this. The DNC maintains that it has done nothing wrong and it has offered the same agreement to all Democratic candidates in the 2016 race.

"The DNC offered to engage in the same joint fundraising efforts with all the major presidential candidates early in the cycle and we welcome the efforts of the candidates to help raise money for the DNC and state parties now to ensure we can build out the infrastructure to win in November," said DNC press secretary Mark Paustenbach.